Iblis was not an angel, he was a Jinn
And when We said to the angels: ‘Prostrate before Adam’, they all bowed down, except Iblis -- he was a jinn and he haughtily disobeyed his Lord. (18:50)
One of a race of beings created before man who were also possessed of Free Will and thus able to make moral choices and elevate themselves in their existence to become closer to Allah (swt) ** This is precisely what Iblis chose to do, and in so doing was around when the creation of Mankind was revealed. There is no record of him ever having being anything other than Jinn, nor of God changing any Angel at all into a Jinn.
Of course, the climb up a mountain of spirituality is pretty hard, and while the view is rather fantastic if you don't watch yourself, and respect how and why you are there, you can quite easily slip and fall. Iblis' demise came when he was asked along with the other angels in heaven to bow down to this new creation of the Almighty.
Then we said to the angels: Bow down to Adam. They bowed down, except Iblis. He was not among those who bowed down. Allah said: ‘What stopped you from bowing down, when I directed you to do so’. He said: ‘I am better than him, you created me from fire and you created him from dust’. (7:11-12)
He chose not to, and ridiculed it on the basis that he was made of fire, while this new thing was inferior as it was made of clay (or mud).
(Allah) said: "Get thee down from this: it is not for thee to be arrogant here: get out, for thou art of the meanest (of creatures)."(7:13)
Now, Allah was not well pleased with this shocking display of racism, vanity, and pride. And immediately condemned the formerly favoured Iblis to the fiery pit of everlasting torment that is Hell.
He said: "Give me respite till the day they are raised up." (7:14)
Iblis, realizing he had just made the biggest mistake of All History asks to be given mercy on the basis that he's been rather wonderful up until now.
(Allah) said: "Be thou among those who have respite." (7:15)
He said: "Because thou hast thrown me out of the way, lo! I will lie in wait for them on thy straight way: "Then will I assault them from before them and behind them, from their right and their left: Nor wilt thou find, in most of them, gratitude (for thy mercies)." (7:16-17)
Allah respites him until the Day of Judgement and Iblis says he will show how unworthy of the Almighty this new creation is by leading them astray.
((Allah)) said: "Get out from this, disgraced and expelled. If any of them follow thee,- Hell will I fill with you all. (7:18)"
and Allah says to him, those who choose to follow you will do so into Hell.
It's interesting to note that in Islam like, in Christianity the devil is someone who previously had a favoured position and then threw it away. It is also interesting to note that Free Will is not unique to Humans, as in Christianity, but also to at least one other intelligent race (the Jinn) created by the Almighty. There is also the associated fact that Free Will is older than Humanity, and that has special implications for the role of the devil.
For a start, it is his denial of the authority of Allah, and his racism, pride and vanity that led to his fall. He was not 'created to be evil' there is no inevitability about evil, and so Free Will is not tainted with influence of 'original sin'. Iblis also chooses to take as many human beings to Hell with him as possible, he acknowledges Allah's omnipotence and also His Mercy in respiting him his punishment to the end of days. Consequently there is a differing motivation for the devil in Islam than in Christianity. The devil isn't gathering evil troops for a final showdown at the Apocalypse, his punishment is inevitable, and his power limited to only those who choose to willfully decieve themselves into following him, and as a consequence denying Allah.
He's taking as many down into the pit with him as he can before the End.
This may help explain some of the profound differences in attitudes towards evil of the two religions as well as account for the variety of motivations currently in play when ideals clash.
** (Allah is the Arabic word for God. The Almighty, The Divine Creator, etc are all used interchangeably.)